A New Method For Analysis Of Re-Os Isotopic System In Organic Rich Sediment

The dating of organic-rich sedimentary rocks not only plays important role in studying the evolution of paleo–environment and paleo-climate, but also has guiding significance in oil and ore detection. However,the existing methods of dating is always indirect, thus gives the result some uncertainty. Re-Os isotopic dating is a way to dating the organic-rich sedimentary directly, but there are still some technological problem in using the method. According to the exsiting research, Rhenium and Osmium in organic-rich sedimentary rocks are dominantly hydrogenous, but any nonhydrogenous component will influence the accuracy and precision of the Re-Os date obtained. Compared with HCl-HNO3 digestion, Cr O3-H2SO4 digestion can minimize the influence of any nonhydrogenous Re and Os, but this medium has a very high reagent blank and is not environmentally friendly. Therefore, we employ a H2O2-HNO3 digestion medium instead of Cr O3-H2SO4.We evaluate whole rock analysis using a H2O2-HNO3 digestion medium together with using HCl-HNO3 and Cr O3-H2SO4 for international standard sample SGR-1b. The date indicate that the H2O2-HNO3 medium with a volume ratio of H2O2 to HNO3 is 5/1 can allow tracer-sample equilibration via complete oxidation of all Os species, and also minimize the influence of any nonhydrogenous Re and Os. Meanwhile, the H2O2-HNO3 medium has a lower reagent blank and is environmentally friendly. The H2O2-HNO3 digestion technique was used in Re-Os dating of the mudstone from Jinxiu, Guangxi Province and carbonaceous shale from Dabao mountain of Guangdong Province. Compared with the date of HCl-HNO3 digestion technique, a more precise isochron age of 404±5Ma with initial 187Os/188 Os ratio of 1.57±0.13(n=8, MSWD=50) and 221±11Ma with initial 187Os/188 Os ratio of 0.67±0.09( n=5, MSWD=11.1)was obtained by H2O2-HNO3 digestion technique, respectively.